1. Field of the Invention
The present invention relates to sensors for process control and, more particularly, to an improved analytical discriminator for high speed spectroscopic constituent analyses.
2. Description of the Background
Many manufacturing processes involve the high-speed application of glue lines to articles such as paper sheets as they are conveyed past an applicator station. Modern automated gluers also include sensors to check the quality of the glue lines and to provide feedback for process control. For instance, in the context of making cardboard packaging, glue lines are applied to container blanks prior to folding. In such process, it is desirable to provide the process computer continuously with real time electrical signals that each glue line has been applied and is of the proper mix of constituents. However, these sensors must support extremely high-volume throughput (often in excess of 1000 sheets per minute). Thus, any analysis of the applied glue must be accomplished in real time, and the need for speed has greatly limited the types of analyses to simple (and largely unreliable) optical checks for the physical presence of glue.
Optical constituent analysis is well-known. For example, U.S. Pat. No. 5,635,402 to Alfano et al. shows a technique for determining whether a cell is malignant or not based on fluorescence. The cell is exposed to a fluorescent dye, it is irradiated, and the intensity of the resulting fluorescence at two wavelengths is compared to known standards to measure the intensity of the fluorescence. However, this fluorescent analyses results in an actual CRT map, and the system is unsuited for any form of high-speed process control or analysis of moving glue lines.
Another more promising type of analysis is based on infra-red reflectance. Infra-red reflectance or spectroscopy has proven itself very useful in other industries and is capable of far more accurate and thorough analysis of constituents in a sample.
For example, U.S. Pat. No. 4,801,804 to Rosenthal shows a method and apparatus for near infrared reflectance measurement of a non-homogenous sample such as ground sunflower seeds. The sample is quantitatively analyzed by uniformly irradiating with near infrared radiation. A bifurcated optical fiber bundle is used with single source and return paths, and various (at least two) wavelengths of the reflected light are successively measured by an optical "chopper" and detector. The wavelengths are ratioed and compared to known values to give a direct reading of moisture content.
Likewise, U.S. Pat. No. 4,840,706 to Campbell shows an infra-red scanning gauge used in measuring the moisture content of a paper-web during manufacture. The scanner employs a measurement channel and a reference channel. U.S. Pat. No. 5,365,067 to Cole et al. shows a method and apparatus for determining surface molecular orientation based on relative intensity of spectral reflectance. The relative intensity is affected by the preferential orientation of the polymeric chains. In both of the above cases, only a fleeting overview of the spectropolarimetry methods are disclosed. There are no structural details, circuit details, or optical details (see Cole '067, column 7, lines 3-6).
U.S. Pat. No. 5,813,403 to Soller measures the pH of tissue by irradiating tissue with 2-20 light sources, measuring the reflectance, and applying a least squares analysis to the measured absorption spectra. This is a pre-dispersant system wherein the specimen is illuminated by various wavelengths. The Soller '403 device is clearly not concerned with speed for process control, and it teaches the use of single optical fibers (see column 3, line 52 et seq.).
U.S. Pat. No. 5,218,206 to Schmitt et al. shows a method for determining the dryness, wetness, or icing of a road. The method employs reflection measurements of light in the infrared range. The reflected light is measured selectively and simultaneously by a receiver in at least two wavelength regions. A quotient of the detected signals determines the respective condition of the roadway surface. The two wavelength regions are selected to ensure that the quotient is indicative of either dryness, wetness., or icing.
U.S. Pat. No. 5,220,168 to Adamski et al. shows a method and apparatus for determining moisture content of materials by irradiating a sample with two wavelengths of light having different water absorptive characteristics (which are therefore reflected by varying degrees depending on the surface moisture on the material). The respective reflections are measured by a single common detector, and a value corresponding to the ambient light is subtracted from each measurement. A ratio of the resultant values is then correlated with data derived from precalibration measurements of samples of known moisture content.
U.S. Pat. No. 5,424,545 to Block et al. shows a non-invasive non-spectrophotometric method for measuring the blood glucose. A plurality of broad spectrum filters transmit beams of radiation in overlapping portions of the spectrum to the sample. Radiation reflected or transmitted by the sample is detected and decoded.
Theoretically, near-infrared reflectance technology is applicable to the context of paper and cardboard packaging as it is capable of substantiating that each glue line has been applied and is of the proper mix of constituents based on the fact that infrared energy is known to be absorbed by typical glues at very specific wavelengths. That is, the absorptivity of infrared energy by glue is known to be dependent on wavelength, Specifically, using the conventional method, paper blanks moving along a conveyor belt would be illuminated under a reflection sensor. The reflected infrared energy power spectrum would be altered according to the characteristics of the glue (such as starch mass). The reflected light would be filtered by two or more narrow band pass filters of different wavelengths, inclusive of a first wavelength that is not readily absorbed by the glue and a second that is absorbed in the glue. By analyzing the relative reflected wavelengths the data is capable of giving a substantive quality check of the glue.
Unfortunately, the conventional analyses required to implement infra-red reflectance techniques as shown in the above-described prior art patents is complex and time-consuming, and there have been few successful efforts to adapt such techniques for the purpose of high-speed process control.
One known example is U.S. Pat. No. 5,663,565 to Taylor, which shows a system for determining glue-line characteristics, such as temperature, of corrugated board. The output signal of an infrared absorption sensor provides an on-line starch measurement for corrugators. The incremental amount of infrared radiation that is absorbed by starch and/or water in the glue-lines is isolated from the predominant, more random background absorption due to cellulose and water in the paper substrate. The amplitude of the extracted signal component, which reflects only the starch and/or water in the glue, is then converted using empirically derived historical data.
Unfortunately, the analysis and implementing hardware used by the Taylor '565 patent is very cumbersome as the data must be compared and converted based on a database of historical data. As shown in the '565 patent, process control speed considerations require that a running average of glue readings be kept over time. The '565 method and device simply is not fast enough to operate in real time to provide a substantive analysis of each running glue line, and it would be greatly desirable to eliminate the need for averaging.